The functional efficiency of shaft seals is frequently impaired by frictional heat evolved at the sealing faces. More especially in the case of seals with a pressure dependent seal surface contact force, there will be a comparatively high frictional power loss at high pressures. If, furthermore, seals are made of materials with a low thermal conductivity there will then be a danger of the maximum allowable temperature for the material being exceeded at the sealing faces so that the seal will prematurely fail.
One expedient used in seal technology for reducing friction losses is the production of an appropriate lubricant film between the sliding seal surfaces to separate them and fill up the seal gap. Such a liquid acting as a lubricant and to be sealed off by the seal may substantially decrease the coefficient of friction. Simultaneously recourse may be had to dynamic film formation also to decrease wear of the sliding seal faces or practically reduce it to zero.
In the case of axially moving piston and rod seals there will be a formation of lubricant film in all cases because of the nature of the system, that is to say, because the parts are wetted periodically by the liquid to be sealed off, and sliding seal parts so move that liquid is entrained into the seal gap.
In the case of sealing shafts a lubricant film is not able to be produced in this way as long as seal interface edge next to the liquid to be sealed is in a plane normal to the axis of rotation of the shaft with the seal: in this case there is no component of motion capable of entraining liquid into the seal gap.
In the case of certain specific types of low-pressure shaft seals, more especially customized radial shaft seals (see Brink, R. V. "The Heat Load of an Oil Seal", paper C1, 6th International Conference on Fluid sealing, 1973, Munich) this problem has been tackled by making the seal edge undulated in the circumferential direction so that it will be possible to produce a component of motion partly directed towards the edge of the seal interface and a consequent hydrodynamic formation of the desired lubricant film. However, such a design was not available for forms of seal ring suitable for use with high pressure differences, as for example o-rings, x-rings or elastomer seals with a rectangular cross section.
Although a proposal has been made in connection with o-rings for example to arrange the seal ring plane obliquely in relation to the shaft axis by having a suitably formed groove in the housing for the o-ring (Leyer, A, "Maschinenkonstruktionslehre", Technica, No. 10, 1960, page 941 ff. and Martini, L, J, : "Slanted o-Rings extend Shaft Life", Machine Design, Feb 8, 1979), such a design did not prove to be practically acceptable, be it because of the complex form and therefore expensive manufacture of the groove or be it because of the unfavorable, asymmetrical direction of the forces and of the flow of heat generated by friction.
For use with hydraulic pressures acting on axially moving machine elements reinforced edge seal packings of reinforced PTFE are frequently used. It might therefore seem obvious to apply the principle, evolved in connection with radial shaft seals, of the axially undulating seal edge to such rings by producing them with an undulating seal edge. Since however as a general rule such PTFE edged seal packings have to be machined, the production of undulating contours would turn out to be an involved and expensive manufacturing operation. In addition there would be an undesired asymmetry of the portions, capable of causing reverse flow, of the dynamic seal faces.
A still further problem in connection with high pressure shaft seals is the risk of the sealing ring, as for example an o-ring or a PTFE edged seal packing, seizing on the cooperating face and then being entrained by the shaft in rotation. Generally this will lead to a premature failure of the seal, inasmuch as the surfaces then making sliding contact with each other will not provide a suitable sliding and sealing function because of unsuitability of the combination of materials or of their surface finishes.